Polymer grade or "purified" terephthalic acid is the starting material for polyethylene terephthalate, which is the principal polymer for polyester fibers, polyester films, and resins for bottles and like containers. Polyester fibers are used in textiles as well as in industrial applications such as tire cord. Polyester films coated with adhesives and emulsions are useful as wrapping tapes, photographic films, recording tapes, and the like.
Purified terephthalic acid is derived from relatively less pure, technical grade or "crude" terephthalic acid by purification of the latter utilizing hydrogen and a noble metal catalyst as described in U.S. Pat. No. 3,584,039 to Meyer. In the purification process, the impure terephthalic acid is dissolved in water at an elevated temperature, and the resulting solution is hydrogenated, preferably in the presence of a hydrogenation catalyst, e.g., palladium on a carbon support, as described in U.S. Pat. No. 3,726,915 to Pohlmann. This hydrogenation step also converts the various color bodies present in the relatively impure terephthalic acid. Another related purification-by-hydrogenation process for aromatic polycarboxylic acids produced by liquid phase catalyst oxidation of polyalkyl aromatic hydrocarbons is described in U.S. Pat. No. 4,405,809 to Stech et al.
Terephthalic acid impurities are of several types. The compound 4-carboxybenzaldehyde (4-CBA), an intermediate product in the oxidation of para-xylene, is found in impure terephthalic acid. Color-forming precursors and color bodies of the benzil, fluorenone or anthraquinone structure, are usually present. Nitro-compounds are found as impurities in terephthalic acid obtained by liquid phase nitric acid oxidation of para-xylene and other suitable starting materials. All of these impurities are deleterious with respect to polyester quality. Any method of purifying crude terephthalic acid to produce fiber-grade terephthalic acid must reduce or eliminate such impurities or convert them to substances inert in the production of polyesters.
Oxidation of paraxylene to crude terephthalic acid is in the presence of a cobalt-manganese catalyst promoted by bromine in an acetic acid-water solution. After liquid-solid separation to obtain the crude terephthalic acid, the mother liquor containing catalyst, acetic acid, 4-carboxybenzaldehyde, color-forming precursors and color bodies and from 1% to 5% crude terephthalic acid, is dewatered. The dewatered oxidation mother liquor can be returned to the oxidation reactor or further treated to recover the catalyst and finally discarded. Return of the dewatered oxidation mother liquor to the oxidation reactor increases the content of 4-CBA, and precursors contributing to color and fluorescence of purified terephthalic acids in the oxidation reactor with consequent decrease in quality of the crude terephthalic acid.
Efforts to selectively hydrogenate impurities and color bodies in the oxidation mother liquor without hydrogenating the terephthalic acid and thus obtain a purified stream suitable for recycle to the oxidation reaction have been typically unsuccessful.